Large concrete products are typically cast in molds. The process of casting is used to make large concrete products such as beams for use in highway bridges, tunnel liners, building construction and the like. Many of these concrete products have tensioned steel strands therein to prestress the concrete product. The steel strands are placed in the mold and tensioned before the concrete is poured in the mold. As the concrete cures, the steel strand and concrete bond and the tension in the strand creates the prestress in the concrete product. Each of the strands is typically tensioned by 30,000 pounds force. Often, the strands are also tensioned perpendicular to their length into a slight V shape near the middle of the mold to provide negative loading at the top of the concrete product.
In a self stressing mold, bearing plates (sometimes referred to as jacking plates) are placed at the ends of the mold and the ends of the strands in the mold pass through aligned holes in the plates and extend outwardly from the plates a length sufficient to allow a hydraulic cylinder or other tensioning device to grasp an end of the strand to tension the strand. Once tensioned, conical wedge type strand chucks acting between the strand and the plates maintain the tension. Because the tension in the strands is passed through the plates, and the plates engage the mold, the tension, in turn, passes through the mold. In such a design, the mold must be sufficiently strong to absorb these stresses.
In other applications, external abutments may be provided at each end of the mold to tension the strands passing through the mold. The external abutments are supported in the ground at the mold site or supported by other structures. In this design, no bearing plates are necessary. The mold is not exposed to the tension forces in the strands and consequently need not be designed to withstand those stresses.
A typical concrete product is a T or double T molded in a long T or double T-shaped mold which may use 2 to 10, or more, tensioning strands in each leg of the T, for example. The mold is often sufficiently long to mold a number of concrete products simultaneously therein along the length of the mold. For example, a mold may be over 400 feet long, and used to mold up to ten 40 foot long T or double T concrete products simultaneously. The ends of the concrete products are formed by headers or bulkheads inserted into the mold at the desired spacing to confine the liquid concrete as it is poured into the mold. The bulkheads are commonly formed of two pieces of ⅜ inch thick plate spaced about 12 inches apart. Each plate forms the end of a particular molded product, with the 12 inch separation between plates so that a worker can get into the space between the plates to cut the strand with a cutting torch or other cutter when removing the products from the form after partial curing of the concrete in the product.
The concrete products are commonly reinforced by rebar or mesh. Commonly, such T or double T molds are self stressing and the concrete products are prestressed by tensioned steel strand passing through bearing plates at the ends of the mold, the headers and the concrete product from end to end, which bonds to the concrete as the concrete cures. The bearing plates hold and distribute the tensioning forces in the steel strand. The bearing plates are typically steel about 4 inches thick to resist the tensioning forces exerted.
Molds for a large, 120 foot long highway I beam, using perhaps 60 separate steel strands, each ⅜, ½, or {fraction (9/16)} inch in diameter, for example, are not self stressing. The strands are drawn through the mold (passing through aligned holes in any headers used) and tensioned between external abutments.
As each strand will usually be at least as long as the mold, say 400 to 500 feet, with some extra length to extend out the ends of the bearing plates or to the external abutments, the difficulty of manipulating such strand lengths can be appreciated. In the past, if 60 strands were needed in the concrete product, 60 separate strand packs could be positioned at the mold site. Similarly, a double T may use 12 separate strands, 6 for each of the two legs of the double T, requiring 12 separate strand packs to be positioned at the mold site.
At present, each strand must be pulled off the strand pack manually and fed first through the bearing plate at the near end of the mold, and then sequentially through each side of the bulkheads in the mold, before finally being fed through the bearing plate at the opposite end of the mold. Generally, the bulkheads are moved close to the near end so that the strands can be manually fed through the near bearing plate and bulkheads with a minimum amount of strand payed off the strand pack. Typically, all the strands to be used in the molding process are manually fed through the near bearing plate and the bulkheads and then a crane or similar device is used to pull the strands and bulkheads simultaneously down the length of the mold, positioning each bulkhead at the proper spacing along the mold and eventually allowing the strands to be fed through the bearing plate at the far end of the mold and tensioned.
The manual operation of feeding the strands through the bearing plates and bulkheads is expensive, time consuming and potentially dangerous. The strand, commonly ½ inch in diameter, is hard to move and manipulate, weighing ½ lb/foot of length. As the strand is payed out from the strand packs, it must rotate or spin to undo the winding of the pack, causing additional difficulties. For a ½ inch diameter strand, the apertures through the bearing plates and bulkheads through which it must be fed are only about ⅝ inch in diameter, leaving little clearance in the installation. It is also critical to avoid cutting or nicking the strands in the installation, as a nick or cut with high tension loading can be the initiation site of a possible strand failure. The molds are often coated with a release agent, making the mold surfaces slippery, and causing additional difficulties for the installation crew.
The apertures in each bearing plate and bulkhead are formed in the particular pattern that the strands will be used in the molded product. For example, for a simple T, there may be six strands in the leg, positioned two to a row in three vertical columns. Occasionally, one strand may be installed in misaligned apertures as it is being fed through the bearing plates and bulkheads, forcing the installation crew to redo the work already done to correct the error, or, if not caught, having an inferior product.
An improved technique is needed to fed the strands through the bearing plates and the bulkheads. The improvement should be less expensive and more reliable than the present manual operation.